The present invention relates to a process and apparatus for the long term shutdown of a high temperature nuclear reactor having a pile of spherical fuel elements in a core by means of a neutron absorbing absorber material.
When shutting down a nuclear reactor, a distinction is made between rapid and long term shutdowns. Rapid shutdowns are effected in the case of accidents which do not permit the continued operation of the reactor and require the rapid discontinuation of operations.
Long term shutdowns take place to discontinue the operation of the reactor in a planned manner and to maintain the reactor in the shutdown condition. The reactivity is usually throttled back in this case more slowly than would be the case with regard to a rapid shutdown.
Depending on the installation, i.e., based on the type and size of the nuclear reactor, the means provided for rapid and long term shutdowns may be similar or different, if safety related redundancy requirements are disregarded.
Different methods for shutdown have been proposed for high temperature nuclear reactors comprising a pile of spherical fuel elements.
In the AVR pebble pile reactor a total of four absorber rods, inserted into the core from below, are used both to control and to shutdown the reactor, with only two absorber rods being required in either instance. The absorber rods are present in and guided by graphite columns, arranged concentrically in the core and surrounded by the fuel elements of the pebble pile. This assures the frictionless insertion and extraction of the absorber rods.
A different configuration is employed in the THTR high temperature reactor, in which the control and shutdown of the reactor is again effected by means of absorber rods.
Rapid shutdowns are carried out in this instance by means of reflector rods of easy mobility, guided in bores in the side reflector. However, their absorption capacity is not sufficient, in view of the dimensions of the reactor core, to maintain the reactor in a subcritical state. For this purpose, a plurality of core rods are provided in a uniform distribution over the cross section of the core. Such core rods, when actuated by adequately powerful drives, are inserted directly into the pile of spherical fuel elements from above. In the process, both the absorber rods and the fuel elements are exposed to strong mechanical stressing as the result of the frictional and inertial resistance applied by the fuel elements to the rod penetrating them. In case of the failure of a drive, the immersion depth of the corresponding rod required for the shutdown is difficult to attain.
DE-OS 32 12 264 describes a layout of absorbers in a high temperature nuclear reactor wherein the rod drives located under the reactor core serve to introduce absorber rods in bores in the side reflector from below.
In a modification of this layout, the side reflector has graphite noses with bores, which project into the fuel element pile and comprise further absorber rods. As a diversionary shutdown device, so-called small absorber pebbles are provided, which may be introduced additionally into the fuel element pile.
Experience has shown that in case of core dimensions exceeding about 2.5 m in diameter and about 5.5 m in height, the effectiveness of the absorbers located in the side reflector is no longer assured for a long term shutdown. Additional absorber are therefore required within the cores.
The solution employed in the AVR fuel pile reactor involves the placement of absorber rods oriented toward the center of the core. A disadvantage is the substantial structural height resulting from the rod drives required, aside from the space needed for the drives themselves.
In the THTR high temperature reactor, absorber rods are inserted directly into the fuel pile. Aside from the fact that here again drives are required which consume a considerable amount of space, the drives must be adequately strong to overcome the high opposing forces resulting from the resistance to penetration of the fuel pile and the friction generated by it. In spite of this, the depth of insertion is limited to approximately 5.5 m by the large forces encountered. The cost of drives of this type is also high.
Direct insertion into the fuel pile also leads to another problem that has heretofore been relevant. As the result of the forces acting during the insertion on the ceramic installations, damage may be caused, such as, for example, pebble fracture, thus resulting in the release of fissionable material.
A further problem may be seen in that the metal sleeves of the absorber rods cannot withstand the high temperatures which may develop in a hypothetical accident.
Finally, it has been shown that it is possible to introduce small absorber balls into the pile of fuel elements. A shutdown of the reactor may certainly be attained in this manner, as a function of the amount of absorber material introduced. However, in reactors with multiple passages of the fuel elements the effectiveness of the balls is restricted by their unfavorable axial distribution in the core.
For the same reason, this shutdown principle is usable only with core dimensions up to a height of 5.5 m.
A further difficulty is encountered in the removal of absorber balls from the core. This procedure is cumbersome and is possible only by circulating the fuel pile while sorting out the absorber balls and the simultaneous insertion of absorber rods, in order to prevent an abrupt rise in the criticality of the reactors. Further, the absorber balls can penetrate into gaps of the bottom reflector of the core and appreciably interfere with the operation of the reactor.
Based on the known state of the art, it is the object of the present invention to provide a reliable process for the shutdown of a high temperature reactor containing a pile of spherical fuel elements, together with an apparatus to carry out the process of the invention, said apparatus being available at all times to shutdown the reactor, without in the meantime, interfering with the generation of power by the reactor. Furthermore, the apparatus should be as cost effective as possible and insure operation over a long period of time without interference.